The invention includes apparatus and methods that enables or enhances capture, release and/or separation of analyte compounds in a sample solution using electric fields and ion current flow through the sample solution channel with cation and/or anion exchange through semipermeable membranes with multiple second solution flow paths. Cations or anions generated or present in the sample solution or in one or more second solution flow paths are transferred through ion selective semipermeable membranes into or out of the sample solution flow channel to effect trapping, electrocapture, binding, displacement, release and/or isoelectric focusing of sample components in the sample solution. The invention comprises a stand alone separation method and apparatus using non mass spectrometer detectors or may be connected to or integrated with an Electrospray Ionization or other Atmospheric Pressure ion source interfaced to a Mass Spectrometer analyzer. The invention apparatus and methods may be scaled to accommodate higher or lower sample solution liquid flow rates.
The invention may be configured in a high pressure liquid chromatography (HPLC) apparatus and methods with packed columns or can be applied to methods and apparatus employing low pressure packed or open tubular column sample separation techniques. The invention comprises one or more semipermeable membrane assemblies positioned along a sample solution flow channel with membrane assemblies separating a sample bearing first solution from one or more second solution flows. Each second solution flow can have a different composition and each second solution composition can change over time using solution composition gradients or step functions. Packed or open channels, where sample component separation occurs, may be configured in a membrane assembly, between membrane assemblies or positioned upstream or downstream of membrane assemblies. Ions generated in the second solution flow paths are transferred through the semipermeable membrane into the sample solution flow driven by the applied electric field. Conversely, ions in the sample solution can be transferred into a second solution through the semipermeable membranes. Ions can be selectively added to or removed from the sample solution flow path at one or more membrane sections positioned along the sample solution flow path. The ion current passing through the semipermeable membrane into the sample solution is subsequently driven along the length of the sample solution channel by a voltage gradient maintained along a portion of the sample solution flow channel length. Individually controlled voltages are applied to electrodes in contact with each second solution flow. Multiple semipermeable membrane sections positioned along the sample solution flow path allow the application of different electric fields and ion currents at different locations in the sample solution flow path. Ions entering the sample solution through one membrane can be remove through an adjacent membrane along the sample solution flow path forming local trapping, capture, release or separation regions.
Configuring one or more semipermeable membrane sections with Electrospray ionization allows independent control and optimization of the capture, trapping, release and/or separation of sample species and Electrospray ionization. Membrane materials and second solution compositions may also be configured to allow selected neutral species to traverse a membrane from a second solution into the sample solution or conversely from the sample solution into a second solution. The transfer of neutral species through a semipermeable membrane is driven or controlled by controlling the relative concentration of the neutral species of interest across each membrane. The added ion or ion and neutral species selectively introduced into or removed from the sample bearing first solution changes the solution pH and/or solution chemistry causing or enhancing binding or release of sample components and effecting separation, cleanup or reactions of analyte species. These processes can also be used to simultaneously optimize or enhance the performance if an Atmospheric Pressure Ion (API Source) such as Electrospray.
Separations of mixtures of analyte components in a solution is widely practiced using packed Liquid Chromatography (LC) separations, open tube Capillary Electrophoresis (CE) and more recently open tube Electrocapture (EC). Liquid Chromatography separation is effected by binding or partial binding of an analyte to a solid phase or material packed within the chromatography column as a liquid or mobile phase flow passes through the column. The liquid flow through the column can be run isocratically, having constant composition, or with changing composition, usually in the form of a gradient or a series of steps. When running isocratic liquid chromatography, analytes are separated in the flowing solution by size differences or differences in partial binding energy with the surface chemistry or phase of material packed within the liquid chromatography column volume. Analyte species that exhibit stronger binding to the column phase will elute from the column at a later time then those analytes with weaker binding energy. Analyte components eluting from a chromatography column are separated both spatially in the solution flow and temporally. When gradient liquid chromatography separations are conducted, the chemistry of the solution passing through the column is varied in a controlled manner to release analyte bound to the column phase material at specific times. Analytes with different binding energies and/or different solution chemistry release conditions will be separated in solution flowing through the liquid chromatography column. Separation of analytes in solution occur due to the partitioning of attractive and release forces on a given analyte species based on the differential interactions between the stationary and mobile phases in a liquid chromatography column. Attractive and release forces are manipulated by changing solution polarity, pH and ionic or buffer species concentration. Analytes are separated in open tube Capillary Electrophoresis and Electrocapture by a balance of electric fields, ion mobility and in the case of Electrocapture, convective solution flow.
The most commonly used types of high pressure liquid chromatography include Reverse Phase (RP), Normal Phase (NP), Ion Exchange (IE) and Size Exclusion (SE) separations. None of these separation techniques are practiced with an ion current passing through the LC column. Capillary Electrophoresis (CE) employs an electric field maintained in the sample solution along an open CE column length to effect separation of analyte species through differential electroosmotic migration of analyte species through a solution with electroosmotic flow (EOF) along the CE column length. Variations in Capillary Electrophoresis have been developed, such as Capillary Electrochromatography (CEC), that combine use electroosmotic separation of CE with the partitioning separation of LC due to differential interactions between two phases. Typically in CE and CEC the sample solution composition remains constant throughout a separation run. Semipermeable membranes have been configured by Severs J. C., and Smith R. D., Anal. Chem. 1997, 69, 2154-2158, at the exit end of CE columns to complete the electrical circuit in a CE-Electrospray Mass Spectrometer interface. Capillary electrophoresis separation of analyte species was used in this apparatus and method. No liquid chromatography separation was described using this interface and no ion species passing through the membrane flowed through the CE column to enhance or cause species separation in solution.
Electrocapture of sample components in solution has been employed to capture sample components in an open column liquid flow stream with subsequent release of components. Electrocapture or analyte species in an open liquid flow column or channel allows preconcentration of samples prior to separation with CE, sample preparation such as desalting, reaction with reagent species and effecting separation of components in the solution flow as described by Juan Astorga-Wells et. al., U.S. Pat. No. US 2005/0284762 A1, Sag-Ryoul Park and Herold Swerdlow, Anal. Chem. 2003, 75, 4467-4474 and Juan Astorga-Wells, Hans Jornaval, and Tomas Bergman, Anal. Chem. 75, 5213-5219. Electrocapture of species is effected using a balance of electric fields, ion mobility and hydrodynamic flow along a liquid flow channel length. Semipermeable membranes separating the sample solution flow from anode and cathode electrodes immersed in reservoirs containing static conductive solutions with no flow have been configured in Electrocapture devices as described in the above references. As described by the authors, sample components in solution have been captured with hydrodynamic forces balanced against an electric field along the sample flow path or on a semipermeable membrane. Release of components can be realized by changing the voltage between the anode and cathode, changing the sample solution flow rate and/or changing the sample solution composition. The Electrocapture devices have no second solution flow to replenish charge in the anode and cathode reservoirs. When the sample flow is stopped, the current in the sample flow channel stops due to charge depletion. In the present invention, second solution flow supplies charged species to the sample solution flow path through the semipermeable membranes and replenishes the charge during operation. The present invention requires no electrolytes added to the sample solution and the electrical current maintained along the sample solution flow path can be changed by modifying the second solution composition with no need to change relative electrode voltage or no requirement to change the composition of the sample solution flow to effect capture and/or release of analyte components in the sample flow.
The use of semipermeable membranes to exchange unwanted ion species in an eluant flow eluting from a liquid chromatography column with a desired ion species has been described previously in EPA Publication Numbers 32,770, 69,285 and 75,371, 69,285 and 180,321 with publication dates Jul. 29, 1981, Jan. 12, 1983, Mar. 30, 1983 and May 7, 1986 respectively. This technique generically described as ion suppression is widely practiced in ion exchange chromatography (IEC) to reduce the conductivity of eluant exiting an IEC column prior to passing through a conductivity detector. Separation is often achieved in IEC by displacing analyte bound to the column stationary phase by charge with a displacing anion or cation added to eluant flow. The anion or cation species, typically added as a net neutral salt, hydroxide or acid compound to the eluant flow passing through an IEC column, can be removed after the IEC column exit by charged species exchange through flat or cylindrical semipermeable membranes as described in the above EPA publications. The selective reduction of solution conductivity without the reduction of analyte species in IEC eluant flow improves the conductivity detection limits of analytes separated while passing through an IEC column. Dual membrane devices are configured wherein the eluant flow exiting the IEC column is in contact with two semipermeable membranes which separate the eluant liquid from two second solutions flowing on the opposite side of both membranes. A voltage is applied between electrodes in contact with both second solution flows driving charged species of one polarity from one second solution flow into the eluant flow while simultaneously driving a charged species in the eluant flow through the second membrane into the second eluant flow. This effectively exchanges charged species in a net neutral eluant flow between the IEC column exit and a conductivity detector. Multiple layer membrane assemblies have been configured to provide exchange or suppression of charged species in eluant flow exiting an IEC column while regenerating the second solution neutral salt, acid or base composition used to exchange ion species.
Similar semipermeable membrane devices have been configured to provide selected cation or anion species with counter ions in aqueous eluant flow as described in U.S. Pat. No. 5,045,204. Two or more membrane assemblies have been configured in contact with the eluant exiting from an IEC column providing the dual function of exchanging ion species to reduce conductivity while simultaneously adding the removed ion species combined with a counter ion to an aqueous solution. The aqueous solution with the neutral salt, acid or base species is used as the eluant flow entering the IEC column as described in U.S. Pat. No. 5,045,204. The exchange of ion species, as described, occurs in an ion exchange resin bed configured downstream of the IEC column exit and after the region of chromatography separation in an ion chromatography system. No ion current passes through the separation or LC column or separation media. In the apparatus and methods described, a net electrically neutral fluid flow passes through the IEC column and no ion current passes through the IEC column. Gradients of ion species with counter ions can be generated post column in the IEC eluant flow exiting the IEC column by increasing the ion current passing through the semipermeable membranes. This is effected by changing second solution composition or the electrical potential applied between electrodes in contact with the second solutions.
The present invention provides the addition and/or removal of charged species through one or more semipermeable membranes to an eluant flow through a liquid chromatography column, an Ion Exchange column, a CE column and/or an Electrocapture flow channel. The invention provides direct ion current through an LC or IEC column or through an unpacked channel. No counter ion is added to the eluant flow and the addition of charged species to the eluant flow through the semipermeable membranes matches the electrical current passing through the LC or IEC column or open sample solution channel length. Charged species added from one second solution into the sample solution eluant flow through a first semipermeable membrane can be removed by passing equal ion current from the sample solution through a second semipermeable membrane into a different second solution flow positioned at the opposite end of the LC, IEC column or open channel. The semipermeable membrane materials and second solution compositions can be selected to introduce charged species and/or organic modifiers into the eluant flow from one or more second solution flows to effect or improve ion exchange, reverse phase chromatographic, CEC or Electrocapture separation of analyte species in solution.
In different embodiments of the invention one or more membrane assemblies are configured with an Electrospray (ES) ion source or other API source interfaced to a mass spectrometer (MS). Such embodiments of the invention can be configured wherein sample separation can be performed integrated with the Electrospray process or the sample separation process can be conducted and optimized independent from Electrospray Mass Spectrometer (ES/MS) processes. Embodiments of the invention provide an efficient and precise means of adding charged species to an eluant flow to effect or enhance chromatographic or Electrocapture separation while simultaneously optimizing Electrospray ionization source mass spectrometer performance. Configuring an Electrospray ion source with a semipermeable membrane assembly whereby Electrospray current is generated in a second solution flow and transferred through the semipermeable membrane is described in U.S. Pending patent application Ser. No. 11/132,953 included herein by reference.